This follows from the migration of the Layer code
done in 99c6e20e69.
In the older `makeLayer` function, the parameters `drawstyle`
and `transparency` could be `None`; in the new `make_layer` function,
`drawstyle` must be an explicit string, and `transparency`
must be a number. If not provided, they use default values.
Move `make_layer` to `draftmake`; `Layer` and `LayerContainer`
to `draftobjects`; `ViewProviderLayer` and `ViewProviderLayerContainer`
to `draftviewproviders`.
The make function and the classes are imported in `Draft.py`
to support the usage of the older `VisGroup`.